Thermal insulation that contains polymeric microfibers has been known for many years. This insulation is commonly used in jackets and sleeping bags to provide heat retention (see, for example, U.S. Pat. No. 5,565,154 to McGregor et al. and U.S. Pat. No. 4,933,129 to Hukman). Microfiber-containing insulation also has been used in shoes to assist in keeping a wearer's feet warm.
Non-woven microfibrous webs, however, are sometimes subject to compression, which can reduce web loft and can cause a reduction in heat retention. In addressing this compression problem, investigators have corrugated nonwoven webs that contain microfibers (see U.S. Pat. No. 5,639,700 to Braun) and have introduced crimped staple fibers into the web (see U.S. Pat. No. 4,118,531 to Hauser). The 3M Company sells such microfiber-containing nonwoven insulation under the brand Thinsulate™.
Molding operations sometimes are used to make shoe footbeds, insoles, or inserts. The molding step enables these shoe products to have a shape that is anatomically adapted to the human foot (see, for example, U.S. Pat. Nos. 4,510,700 to Brown and 4,932,141 to Hones).
Shoe footbeds also have been developed which use a layer of thermal insulation to protect the wearer's feet from cold ambient temperatures (see, for example, U.S. Pat. No. 4,464,850 to Ebert et al. and U.S. Pat. No. 4,658,515 to Oatman) Known molded shoe footbeds, however, have not used thermal insulation that contains nonwoven webs of polymeric microfibers.
Processing articles that contain non-woven polymeric microfibrous webs can sometimes be problematic. Because polymeric microfibers are subject to changes in morphology and structure when subjected to heat for only short time periods—and because some molding operations occur at temperatures at or above the melting temperature of the microfibers—the web and its heat retention capabilities may become damaged during subsequent molding operations. Manufacturers accordingly tend to avoid using low melting point, polymeric, microfiber-containing webs in operations where web loft and fiber integrity need to be maintained.